Radio receiving system



May 16 1933. J w 7 1,909,239:

RADIO RECEIVING 'sYsTEM Filed June I930 FREQUE/VC Y IIZQVTOR. 1 BY 1ATTORNEY I Patented May is, 1933 UNITED STATES PATENT OFFICE I CHARLESTRAVIS, OI PHILADELPHIA, PENNS YLVANIA, ASSIGNOB 'IO ATWA'IBBIANUFAG'IUBING COMPANY, OI PHILADELPHIA, PENNSYLVANIA, A

TION OF PENNSYLVANIA RADIO RECEIVING SYS'JPE H Application fled June 7,

My invention relates radio receiving systems, and particularly. to thedetection or demodulation of the received radio-frequency energy aseifected by a thermionic detector ond impedance is interposed betweenthe anode .of the detector tube and ground, and

the anode and the cathode of the detector tube connected to the grids ofthe amplifier tubes. a Preferably the anode circuit direct-current flowsthrough the resistance, and all or any desired part of the voltage dropacross it, is utilized to bias the grid of the detector tube to'suitablenegative value with respect to the cathode for plate circuitrectification. To prevent the grid from varying at audio-frequency andto overcome the disadvantage of plate-circu' rectification that increaseof radio-frequency input above a certain value results in decrease ofaudio-frequency output, the grid return is connected to the cathodethrough a condenser of low impedance to radio and audio frequencycurrents, and to the coupling resistance through a suitably highresistance, the condenser and high resistance comprising a network whosetime constant is greater than the period of the'lowest audio-frequencyin the detector output.

My invention further resides in the methods and systems hereinafterdescribed and claimed. 1

For an understanding of my invention-and for illustration of some of theforms it may take, reference is to be had to the accompanyin drawing inwhich: I v

Fig. 1 represents diagrannnatic'ally a radio receiving apparatusutilizing my invention.-

' tube V2, throughthe radio frequency choke.

L2 and the blocking condenser C2 of 100 v Figs. 2 an 3 are explanatorycurves. 1 Fig 4 represents diagrammatically, utiliz- 1930. Serial No.459,848;

ing a push-pull'resistance coupled amplifier in the output circuit of adetector Referring to Fig. -1, radio frequency energy received by theantenna A is impressed upon the input circuit. LC of a detector tubethere being preferably one or more stages radio frequency amplificationinterveningslider 8 along the resistance Ti, connected in the antennapath controls the amplitude of the energy impressed upon the detectortube and therefore the volume of signals reproduced by apparatus, as aloud s eaker S associated with the tube system 0 .the detector. In sofar as the invention herein described and claimed is concerned, thevolume of reroduced signals may be varied in any other nown way: forcontrolling the amplitude of signal energy delivered to the detectorinput circuit.

The positive terminal of the power supply B, which may be a battery or aB eliminator, is connected to the anode a of the detector tube and thenegative terminal is connected to the resistances R1, R2, in series, tothe cathode f of the tube. While for convenience separate batteries havebeen shown, it will be understood that the same source may supply theanode current for all tubes of the apparatus. The grids g of the tubesare at a potential always lower than that of or is negative with respectto, the associated cathode 5 by virtue of the voltage across resistancconnected between the cathode f and the negative terminal of the Bbattery. The negative biasing for theradio frequency amplifier tube V1and the audio frequency amplifier tube V2 may be obtained in any otherknown way. The negative bias for the grid of the detector tube V isderived asv more fully hereinafter described.

The cathode -f of the detector tube or the 7 high potential end of theresistance R1 is connected to the grid 9 of the audio amplifier coilsuitable low impedance for currentsi cf audio frequency. The lowpotential end of resistance R2 is connected to the cathode of the audiofrequency amplifier so that the resist ances R1, R2 in series,constitute an input element for the audio amplifier tube, the radiofrequency b -pass condenser 03 particularlyin combinatlon with the chokeL2, preventing the potential of the grid 9 of the audio amplifier tubefrom varying at radio frequencles. The condenser C4 is of low impedancefor currents of audio frequency so that the anode a of the detector tubeis at ground potential for audio frequency.

.The impression of'modulated radio frequencysignal energy upon the inputcircuit of the detector tube results in-the flowof an audio frequencycurrent corresponding to the modulations of the radio freqnency energy,through the resistances R1, 2, producing across the resistances acorresponding audio frequency voltage which is impressed upon the inputelectrodes of the audio amplifier tube V2 in whose output circuit isincluded the reproducing apparatus, as the loud speak- "ei- S; Asindicated late current may be supplied to the ampli er tube V2 throughthe audio frequency choke L3, the condenser C5 affording a path to thespeaker of low edance for the audio frequency variations 0 the anodecurrent.

The flow of direct current through the re- 0 sistances R1, R2 isutilized to bias the grid 9 of the detector tube V. i To that end thelower terminals of the inductance L and tuning condenser C are connectedto a suitableipomt in resistance R1 or R2, or asindicated to a pointbetween them. The by-pass condenser, C6, afiords a path of low impedancefor, radio frequency currents directly to the cathode of the tube. Theresistances R1, R2 therefore,

have the double purpose of coupling the input circuit of the audio amlifier to the out ut circuit of the detector tn and of providmg ne ativegrid bias for the detector tube.

the operation of radio receiving apparatus using anode circuitrectification, it has been found that, as shown graphically in abscissaof the point the amplitu e of volume of the reproduced signals aseflected by a loud speaker or the like, is the same for both the weakerand strongersignals, or assuming a certain amplitude of received signalenergy, and a certain setting of a volume control, such as for example,the contact a on resistance R, the audio output may have amagnitudecorresponding with the ordinate of the point A or the audiooutput may correspond with the ordinate of the point B as when thecontact 8 be moved to some other position corresponding with materiallygreater signal voltage impressed upon the detector input.

A v di s dvantage of this characteristic of detectors of'the typereferred to, is that the qllliality of reproduction corresponding with te stronger signals, for which the audio output has the magnitudeindicated by the portion Y of the curve is poor, or at least inferior tothat correspondin with operation upon the portion X of the caracteristic curve.

By suitably controlling the grid bias of the detector tube, theradio-input audio out-- put characteristic curve of Fig. 2 is modifiedto comprise the portion X, as before, with the portion Z for allpointson which, at least within the vicinity of the upper end of Curve Xand-materially beyond, the audio output does not decrease with increasein radio input; and accordingly the situation does not arise that fordifferent magnitudes of radio input there may be the same magnitudes ofaudio output.

Furthermore, slumping of the radio-input audio-output characteristicbeyond the region :1: also gives rise to the phenomena of-an a parentdouble peaked resonance curve. s illustrated by Fig. 3, when the inputcircuit is tuned to a frequency F and the signal energy is of suchstrength that due to resonance the radio frequency potential impressedupon the input circuit of the detector is at its maximum and correspondsto a point on the poition Yof the curve of Fig. 2, the audio outume ofreproduction, is substantially less than that of signals to which theset has not been tuned, but which give rise to radio frequency voltagesin the input circuit which correspond with points upon the portion X ofthe characteristic curve.

Inasmuch as the operator in tuning a set makes an adjustment whichefiects maximum amplitude of reproduction, he would not, as indicated byFig. 3, tune to the frequency F, of the desired signal, for he would notknow when his adjustment corresponded with resonance for the frequencyF, since the audio output or amplitude of reproduction is at maximum forsome other frequency, such as F or F, either higher or lower than thefrequency F. In seeking maximum response, the o erator therefore,hasunconsciousl detuned his set away from frequency F, wit the result as reards the desired signal, he has impaired ua ity and increased thelikelihood of inter erence from other stations.

.put of the detector, and therefore, the vol- These difliculties may beovercome by selecting a. suitable value for the high resistance B3- Thetime constant of the network,

comprising the condenser C6 and resistances condenser G6 a capacity of.1 microfarad and resistor R2, external to the network, a value of theorder of 25,000 ohms. Although the condenser C6 is of low impedance foraudio frequency currents, it is in series with high resistanceR3 acrossresistance R1, so that substantially all of'the audio frequencycomponents of the detector anode current flows through resistance R1.The grid of the detector tube because of the low impedance of condenserC6, and the high magnitude of resistance R3 is, for audio frequencies,at the same potential as the cathode although its direct currentpotential is negative with respect to cathode.

Due to asymmetrical conductivity between the grid 9 and the cathode,when for Strong radio input voltages the grid is positive for a smallfraction of a cycle, there is produced in the input circuit a smalluni-directional current component or uni-directional current impulses,which by virtue of the network is a substantially direct or continuouscurrent, effecting between the grid and cathode a potential differencewhich negatively biases the grid, having the effect that thecharacteristic curve of Fig. 2 is converted from the combination -X, x,Y to X, Z, with the result,

previously indicated, that for very strong incoming signals, or forsuitable manipulation of the volume control, the audio output does notslump or decrease; and with the further result that maximumaudio outputoccurs at resonance, if the input system be tunable t0 the receivedsignal energy.

This bias is in addition to the normal biasderived by flow of the anodecurrent through the resistance R1. The direct or continuous grid currentdue to the received signal energy and traversing resistance R3 is alwaysofsuch small magnitude that the selectivity of the tunable circuit L3 isnot appreciably impaired. This automatic negative grid bias ond, whichcorresponds to the period of a current of a frequency of 10 cycles persecond, and is therefore greater than the period of lowest frequencysought to be preserved in the reproduction of sound.

The normal bias of grid g in the example given may be about 16 volts,the drop across resistance R1, but upon reception of a strong signal thebias is automatically increased by the network to as hi h as 30 volts,for example. The negative iasefiected' by the received' signal energyitself, does not instantly disappear upon cessation of that energy, butdecreases to a certain fraction of its former magnitude within a certaintime, which is the If, therefore, the time constant were made too great,as for example of the order of several seconds, the receiving set wouldbe for such length of timeout of condition to receive weak signals towhich, within such period, the operator might seek to tune the receivingsystem. From the standpoint of speed of use,

period or time constant of the network.-

it is therefore desirable that the time constant of the network be not:to great, and on the other hand, it is desirable that it be not tooshort, that is, shorter than the period of the lowest frequency ofmodulation.

The anti-slump filter network is claimed in my co-pending applicationSerial No. 472,378, filed August 1, 1930, in which it is disclosed incircuit with a detector whose cath ode is at substantiallyZgroundpotential.

In the modification of my invention, as shown in Fig. 4, the detectortube V'is directly resistance coupled to the push-pull amplifier tubesV2, V3; The audio frequency bypass condensers C4 connecting the anode ato earth is omitted and there is substituted therefor a radio frequencyby-pass condenser C7. The anode of the tube is connected through theradio frequency choke L4, generally similar to L2, and a blockingcondenser C8, generally similar to the condenser C2, to the grid of asecond audio amplifier tube V3. The resistance R4 connected-for examplebetween the positive terminal of the B battery and the anode ofthedetector tube is, in magnitude moreor less equal to the sum of the.resistances of .the'resistors R1, R2, fomexample, of the order of100,000 ohms. The cathodes f of the amplifier tubes are as indicated,connected to earth or equivalent E, so that the resistance R4 iseffectively in the input circuit of the tube V3 and the resistances R1and R2 in series are in the input circuit of the tube V2. As indicatedthere may be provided a grid. leak resistance R5, whose oppositeterminals are connected to the grids of the an plifier tubes, and to anintermediate point of which connection is made to earth.

In the arrangement described, impression ofmodulated radio frequencyenergy upon the in ut circuit of the detector tube causes flow of audiofrequency current through the resistances R1, R2 and R4 in series. Boththe anode and cathode of the detector tube are of audio frequencypotentials substantially different from that of the cathodes if thetubes V2 and V3, and which potentials feet in opposite sense the gridsof the pushpull amplifier tubes, whose output circuit in.-

cludes the usual transformer T provided with a split primary P and a'secondary S1 for connection to a loud speaker or "other translatingdevice.

As in Fig. 1 the grid of the detector tube may be biased by voltagederived from flow of anode current through the resistance R1, andadditionally by grid current flowing in the resistance R3.

It will be understood that in the modifications shown the electrodecircuits of all tubes may be supplied from a common source ofalternating current by means of proper'transformers, rectifiers, filterchokes and other accessories as commonly employed in present alternatingcurrent operated sets. The filaments or heaters of all tubes may besupplied with current from the same secondary winding of a transformerwithout in any wa interfering with the proper operation 0 the circuits.

What I claim is 1' 1. In radio receiving apparatus, a detector tubehaving anode, cathode, and grid electrodes, a resistance connectedbetween said cathode and earth and traversed by low frequency currentsin the anode circuit of l. v o a said tube, a connection of lowimpedance to low frequency current between the anode of said tube andearth to increase the low frequency drop of potential across saidresist? ance, and a translating device responsive to low frequenciesincluding said resistance in its input circuit.

2. In a radio receiving apparatus, a detector tube having anode, cathodeand grid electrodes, a resistance connected between said cathode andearth, a condenser in shunt thereto of low impedance to currents ofradio frequency, a path of low impedance to currents of low frequencybetween the anode of said tube and earth for increasing the lowfrequency drop of potential across said resistance, and a translatingdevice responsive to low frequencies including said resistance in itsinputcircuit. a r

3. In a radio receiving apparatus, a detector tube having'ano'de,cathode, and grid electrodes, a source of direct current anodepotential, a resistance connected between said cathode-and the negativeterminal of said source, a connection from a point of said resistanceremote from the cathode to'said grid to effect negative biasing thereof,a condenser in shunt to said resistance of low impedance to currents ofradio frequency, a condenser between said anode and the. low

potential end of said resistance of low-imtial across said resistanceand a trans ati mionic detector tube having anode, cathode,

and grid electrodes ,'a'res1stance connected between the cathode of saidtube andearth, a second resistance substantialIy equal in magnitude tosaid, first resistance and connectedgb etween the anode of said tube andearth, afidm'pair of audio amplifier tubes having their cathodesconnected to earth and their grids connected respectively to the cath-.89

ode and anode of said detector tube:

5. In radio receiving apparatus, athermionic detector tube having anode,cathode, and grid electrodes, a resistance connected between-the cathodeof said tube and earth,

a second. resistance substantially equal in magnitude to said firstresistance and connected between the angde of said tube and earth, apair of thermionic amplifier tubes having their cathodes connected toearth and their grids connected respectively to the cathode and anode ofsaid detector tube,-and a common source for rendering the cathodes ofsaid detector and amplifier tubes electron emissive.

6. In radio receiving apparatus, a thermionic detector tube havinganode, cathode and grid electrodes, a source of direct current anodepotential, a resistance connected etween the negative terminal of said"source and the cathode of said tube, a second resistance substantiallyequal'in magnitude to said first resistance and connected between thepositive terminal of said source and th e anode of said tube, a pair ofthermionic amplifier tubes having their grids connected respectively tothe cathode and anode of said detector tube and connections frona theirplates and cathodes to the positive an negatiye terminals of said sourcerespec- '110 tively.

7. Inradio receiving apparatus, a det ector tube having anode, cathode,and grid electrodes, a source of direct-current anode potential, aresistance between the negative minal to the grid of said tube, acondenser offering small impedance to currents of radio frequencyconnected between the cathode of said tube and the other terminal ofsaid input element, a direct current'path from said other terminal to apoint of-said resistance removed from the cathode to effect biasing ofsaid grid, and a translating deviceresponsive to audio frequenciesincluding said resistance in-its input circuit.

8. In radio receiving apparatus, a detector [186 tube having anode,cathode, and grid electrodes, a source of direct-current anodepotential, a resistance between the negative terminal of said sourceandsaid cathode traversed by the audio frequency and direct currentcomponents of the detector anode current, an input element connected atone terminal to the grid of said tube, a condenser oifering smallimpedance to currents of radio frequency connected between the cathodeof said tube and the other terminal of said input element, a directcurrent path from said other terminal to a .point of saidresistance-removed from the cathode to effect biam'ng of said grid, asecond resistance included in said'path and cooperating with saidcondenser to prevent fluctuation of the grid potential ataudio-frequency and to modify the biasing potential on the grid inaccordance with the amplitude of the signal, and a translating deviceresponsive to audio frequencies including said first resistance in itsinput circuit.

9. In radio receiving apparatus, a detector tube having anode, cathode,and grid electrodes, a source of direct-current anode potential, aresistance connected between the negative terminal of said source andsaid between the cathode of said tube and the negative terminal of saidsource and traversed by the audio-frequency and direct cur rentcomponents of the detector'anode current, a condenser of low impedanceto audiofrequencies connecting the grid of said tube to the cathodewhereby the grid is of substantially the same audio-frequency potentialas the cathode, a resistance connecting the grid of said tube to a pointof said first resistance whereby the gridis negatively biased withrespect to cathode by the direct current component of the detector anodecurrent, said second named resistance and condenser cooperating to varythe grid-biasing potential in accordance with the amplitude of receivedsignal energy, an audio amplifier tube, and connections of low impedanceto audio frequencies from the grid and cathode of said amplifier tube tothe cathode of said detector tube and the negative terminal of saidsource respectively, for impressing on the input electrodes of theamplifier tube an audiofrequency potentialderived from theaudiofrequency component of the detector anode current.

CHARLES TRAVIS.

for modifying the grid biasing potential and whose time constant isgreater than the period of the lowest frequency of said audio frequencycomponent of the anode current.

,10. In radio receiving apparatus, a detector tube having anode,cathode, and grid electrodes, a source of direct current anode 'current,'a-resistance connected between the negative terminal of said source andsaid cathode and traversed by the audio frequency and direct currentcomponents of the detector anode current, a path in shunt to saidresistance including a condenser and a second resistance in series. saidcondenser and second resistance comprising a network for varying thegrid biasing potential in accordance with signal amplitude and whosetime constant is greater than the period of the lowest frequency of saidaudio frequency component of the anode current, means for impressingmodulated radio-frequency energy upon the input-circuit comprising aninput element connected from the grid to a point in said 66 shunt pathbetween said condenser and said second resistance.

